With the rapid advance of organic electronics and optoelectronics such as organic light emitting diodes (OLED), organic field effect transistors (OFETS) and organic photovoltaics (OPV) cells, electrical doping of organic thin films has been recognized as a means to overcome fundamental material properties limiting their electrical performance. Electrical or chemical doping of molecular organic films is an efficient means of improving and controlling charge injection and carrier transport in organic devices. Doping can enhance device efficiency by introducing a space-charge layer that improves charge injection via tunneling and by providing additional free carriers to increase carrier density and mobility. The most commonly used n-dopants in molecular devices are alkali metal atoms (i.e., Li or Cs atoms) that donate an electron to the molecular host (Kido et al. (1998) Appl. Phys. Lett., 73:2866; Parthasarathy et al. (2001) J. Appl. Phys., 89:4986; Yan et al. (2001) Appl. Phys. Lett., 79:4148; Mason et al. (2001) J. Appl. Phys., 89:2756; Le et al. (2000) J. Appl. Phys., 87:375; Ihm et al. (2003) Appl. Phys. Lett., 83:2949; Liu et al. (2004) Appl. Phys. Lett., 85:837). While alkali metals have the appropriately low ionization energy to n-dope practically any organic material (Kido (1998) Appl. Phys. Lett., 73:2866; Gao et al. (2003) Chem. Phys. Lett., 380:451; Ding et al. (2005) Appl. Phys. Lett., 86:213508), these materials present several intrinsic limitations. First, alkali metal atoms have small atomic radii and are prone to diffusion through the organic film. This makes it difficult to produce well-defined space-charge regions, introduces device instability, and causes undesired quenching sites at light-emitting interfaces. Second, the small positive counterion that remains after charge donation (e.g., Li+) lies close to the host molecule and creates a large coulombic trapping potential for the donated electron. Third, this form of n-doping is accompanied by decomposition of the organic transport material (Le et al. (2000) J. Appl. Phys., 87:375). Alkali metals are, therefore, relatively inefficient and undesirable donors in molecular solids.
Molecular doping, that is, electrical doping using molecular compounds, is viewed as a possible solution to the limitations listed above. A bulkier organic molecule would reduce or eliminate dopant diffusion and larger organic-organic molecular distance would minimize the trapping of donated carriers by the ionized dopants. Although p-type molecular doping (e.g., with fluorinated tetracyanoquinodimethane (F4-TCNQ)) has been investigated and applied (Blochwitz et al. (2001) Org. Elect., 2:97; Gao et al. (2002) Org. Elect., 3:53; Gao et al. (2003) J. Appl. Phys., 94:359; Gao et al. (2003) J. Phys. Condens. Matter, 15:S2757-S2770; Chan et al. (2004) J. Vac. Sci. Tech. A, 22:1488), the energetic requirements for molecular n-type doping have hindered the identification and development of suitable electron donor/acceptor pairs. Since the electron affinity (EA) of most organic electron transport materials is smaller in magnitude than ˜4 eV (Kahn et al. (2003) Polym. Phys., 41:2529), the ionization energy (IE) of an efficient organic n-type dopant needs to be equally small. However, such materials are easily oxidized and generally unstable under ambient conditions. Indeed, the synthesis, handling, and delivery of larger organic dopants with sufficiently low ionization energy (IE) for efficient electron transfer to most host materials of interest, have proven very difficult and impractical (Nollau et al. (2000) J. Appl. Phys., 87:4340; Wang et al. (2006) Chem. Phys. Lett., 423:170; Werner et al. (2003) Appl. Phys. Lett., 82:4495; Werner et al. (2004) Adv. Funct. Mater., 14:255; Chan (2006) Adv. Funct. Mater., 16:831). For example, the organic salt precursor pyronin B chloride was found to decompose under thermal evaporation to produce a neutral radical capable of n-doping a material like 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) (Werner et al. (2003) Appl. Phys. Lett., 82:4495; Werner et al. (2004) Adv. Funct. Mater., 14:255; Chan et al. (2006) Adv. Funct. Mater., 16:831). However, this type of compound is relatively difficult to use and the complex chemical and physical interactions with the host ultimately limit their utility in electronic devices (Chan et al. (2006) Adv. Funct. Mater., 16:831).